It can totally depend!
I've been there in December when it's been nice during the day (capri's, not shorts weather really) and my kids swam. I've been there when the high is 50 degrees F during the day, and in the 30's overnight, so we needed jackets, even once a winter coat.
I'd say to plan for anything, then check the weather forecast the week before you go. You'll definitely need at least a light jacket for evenings.
Like the others say, the pools are open & heated (not enough for me, though!). Many resorts have hot tubs as well.